The present invention relates to the field of aberration compensation in image projection for electronic image display or image sensing. The invention is applicable, for example, to optical projection displays used for viewing electronically produced and electronically reproduced images, graphics and text, including video and television images. In particular, embodiments of the invention relate to the arrangement of imager geometries for compensating projection lens aberrations in projection displays. Applications for optical projection systems include computer monitors and televisions.
In a typical optical projection display, images are produced by an image-forming device (sometimes referred to as an xe2x80x9cimagerxe2x80x9d) having electronically addressable picture elements. A picture element, or pixel, is the smallest independently addressable area of the imager, and is the xe2x80x9cunit cellxe2x80x9d from which images are constructed. Pixels are most commonly aligned in horizontal rows and vertical columns, but other geometries are sometimes used. Imagers may comprise, for example, liquid crystal cells, tilting micromirrors, light emitting diodes, lasers, or other devices which produce, modulate, direct, alter polarization, or otherwise modify light. Imagers may be illuminated by an external source of light such as an arc lamp, a light emitting diode (LED) or a laser source. Illuminated imagers may be reflective or transmissive. Imagers may also be self-luminous, comprising arrays of LED""s, lasers or the like. Color display systems may include three separate imagers producing three different colored sub-images in the image to be displayed, for example producing red, green and blue sub-images which combine to form the complete image in an RGB color additive system. Optical projection displays also comprise imaging optics which collect light leaving the imager and form a magnified image on a viewing screen. The imaging optics normally include a projection lens, and may also contain a light source, illuminator optics, beam-splitters, filters, polarizers, retardation plates, mirrors and the like.
In prior art optical projection systems, an imager is normally constructed with an ideal pixel geometry. The ideal pixel geometry is geometrically similar to the ideal screen geometry, where the ideal screen geometry is the desired geometry of the projected image on the viewing screen. Geometric similarity means that all dimensions on the ideal screen geometry are in a constant ratio to their corresponding dimensions on the ideal pixel geometry, that ratio being the magnification of the projection optics. An example of an ideal pixel geometry is a uniform rectangular grid of identical rectangular pixels. An imager with an ideal pixel geometry would produce an image with an ideal screen geometry if aberration-free imaging optics could be used. In practice, real imaging optics have aberrations which cause geometric errors in the projected image. Thus, an imager with an ideal pixel geometry does not generally produce an image with an ideal screen geometry.
In prior art systems, optical designers have attempted to design nearly-aberration-free imaging optics which produce images with nearly-ideal screen geometries when used with imagers having ideal pixel geometries. For many prior art optical projection display designs, this has been an exceedingly difficult or unachievable task. Previous attempts to solve aberration problems have concentrated on designing increasingly complex projection lenses, for example comprising expensive low-dispersion glass types. In particular, the difficulty of designing imaging optics with sufficiently small lateral chromatic aberration has increased the design cost and reduced the image quality of many existing optical projection display designs. This problem has also limited the designer""s ability to create new designs with wider screens, smaller enclosures, sharper images, lower cost, and other desirable features.
Aberrations in the imaging optics which cause significant geometric errors in the projected image may include lateral chromatic aberration, radial distortion and keystone distortion, each of which is discussed briefly below.
Lateral Chromatic Aberration
In an imaging system, lateral chromatic aberration is the variation of magnification with wavelength, where magnification is the ratio of an image dimension to a corresponding object dimension. In a properly aligned optical projection display, the red, green and blue (RGB) sub-images will coincide at the optical axis. Lateral chromatic aberration in the imaging optics causes registration errors between the RGB sub-images that increase from zero at the projection lens axis to a maximum at or near the corners of the image. Although the optical designer attempts to minimize lateral chromatic aberration when designing the imaging optics, an uncorrected residual aberration invariably remains. The design target for the maximum allowable RGB sub-image registration error in high quality displays may be xc2xc pixel or less at the corners of the projected image. However, the registration error between the RGB sub-images due to chromatic aberration is commonly larger than the pixel size, even in xe2x80x9cbest effortxe2x80x9d designs. Thus, the desired level of lateral chromatic aberration correction may not be achievable in the design of imaging optics having acceptably low cost and complexity. The inability to control lateral chromatic aberration often limits the image quality attainable in optical projection displays.
Radial Distortion
Third-order distortion causes a displacement of image points toward or away from the optical axis by an amount proportional to the cube of the distance from the optical axis. Positive third-order distortion causes a square object to form a pincushion-shaped image. Negative third-order distortion causes barrel-shaped images. Fifth-order distortion is similar, except it has a fifth-power dependence on distance from the optical axis. Third, fifth, and higher order distortion are collectively referred to as radial distortion. It is not always practical to eliminate radial distortion in the design of the imaging optics.
Keystone Distortion
Keystone distortion occurs when the imager and the viewing screen are tilted with respect to the optical axis of the imaging optics. As its name implies, keystone distortion causes rectangular objects to produce trapezoidal images. In compact optical projection display designs, it may be desirable to project the image onto a tilted viewing screen using the Scheimpflug condition to ensure a well-focused image. Since keystone distortion lacks rotational symmetry, it is not normally correctable in the design of the imaging optics.
In accordance with the principles of the present invention, there is provided an optical imaging system having an imager arranged to project or receive imaging light through imaging optics having at least one lens or concave mirror. The imager is constructed to include an array of pixels in which the pixel geometry is arranged to compensate for aberration in the optical system to thereby provide corrected imaging.
The optical imaging system may be in the form of a projection display system, or may be an image acquisition system such as a digital camera or the like.
The optical imaging system can include a plurality of imagers which project or receive light of respective different colors through the same imaging optics.
In one form of the invention the pixel geometries of the imagers are arranged differently to compensate for lateral chromatic aberration. Further, the pixel geometry of each of the imagers can be arranged to compensate for radial distortion and/or keystone distortion aberrations.
In order to produce color images, the optical imaging system may include a plurality of imagers, wherein a first imager produces a first image field in a first wavelength range and a second imager produces a second image field in a second wavelength range. Preferably the first imager has a different pixel geometry than the second imager. The optical imaging system may further include a third imager which produces a third image field in a third wavelength range, the third imager preferably having the same pixel geometry as the first imager.
The present invention also provides a pixelated imager having a two dimensional array of controllable light producing, modifying or sensing picture elements, wherein the array has a non-uniform geometry with the picture elements arranged to compensate for optical aberrations of associated imaging optics. In a particular form of the invention, the spacing of pixels in the picture element array comprises a variable number of photolithographic grid spacings.
In accordance with the present invention there is also provided a method for producing an imager for an image forming or image acquisition system which projects or receives imaging light through imaging optics. The method includes selecting an optical imaging system configuration for use with an imager, and designing the imaging optics for use with the imager in the selected configuration. The method also includes selecting a set of start points, and mapping a set of end points corresponding to the start points projected through the imaging optics. A pixel geometry is designed for the imager using the mapping of the start points to the end points, and an imager is produced having the designed pixel geometry. The imager having the thus designed pixel geometry has pixels placed so as to compensate for aberrations in the imaging optics.
Preferably the mapping of the end points from the start points is accomplished by computerized ray tracing.
Where the optical imaging system configuration is an image projection display having a projection screen, the selected set of start points preferably correspond to a set of points on the projection screen. Preferably the set of end points correspond to a set of points in the plane of the imager in use in the selected optical imaging system configuration.